1. Field of the Invention
The present invention relates to a method for producing acrylic acid. More particularly, it relates to a method for producing acrylic acid which adopts a crystallization process and an acrylic acid dimer decomposition process in the purifying step.
2. Description of the Related Art
Acrylic acid is used chiefly for producing acrylate that is an important raw material for textile fibers, surface coating materials, dispersants, and adhesives. In addition, recently, the use of acrylic acid for the raw material of highly water absorber has increased, and a highly purified acrylic acid having an impurity concentration of tens to hundreds of ppm by weight is required.
The propylene oxidizing method by which propylene and/or acrolein are oxidized in catalytic vapor phase oxidation is industrially common for the method of producing acrylic acid. When acrylic acid is produced by the propylene oxidizing method, in the oxidation process of propylene impurities such as water, acids such as propionic acid, acetic acid, and maleic acid, acetone, and aldehydes such as acrolein, furfural, and formaldehyde are by-produced. A gas containing these by-products is absorbed as an acrylic acid solution generally by contacting the gas with a solvent, and the resulting acrylic acid solution is purified by separating the solvent by the method of distillation, etc. and then additionally separating the light boiling point components and the high boiling point components.
In order to obtain a more highly purified acrylic acid from the thus obtained acrylic acid, trace impurities such as aldehydes which cannot be easily separated by distillation are separated by use of a treatment with chemical agents, and a distillation process following the treatment or crystallization process. However, in the above-mentioned method there are too many processes to produce the highly purified acrylic acid, and the apparatus and the operations therefor are also too complex. In addition, there is a drawback that the load of high temperature is put on the distillation of acrylic acid, and therefor the dimerization, and oligomerization, orpolymerization of acrylic acid occurs to decrease the yield of acrylic acid.
JP-A-9-227445 discloses a method of producing the highly purified acrylic acid by absorbing a gas containing the acrylic acid obtained by the catalytic gas phase oxidation with a high boiling point solvent, separating the absorbed mixture into a solvent and a crude acrylic acid by means of distillation, and then using a crystallization process. However, this method cannot also avoid reducing the yield of acrylic acid because of the occurrence of the dimerization of acrylic acid in the absorption process and the distillation process.
Therefore, an object of the present invention is to provide a method of producing the acrylic acid having a high purity in a high yield.
We have diligently studied the method of producing acrylic acid, and as a result have found that the above described object is attained by oxidizing propylene and/or acrolein by means of catalytic vapor phase oxidation, subsequently causing the obtained reaction mixture to be absorbed in a solvent, distilling the obtained product to obtain a crude acrylic acid from the column bottom portion and/or the column side portion, purifying the crude acrylic acid by use of a dynamic and static crystallization steps, feeding a residual mother liquid to a acrylic acid dimer decomposition step to collect acrylic acid, and returning the collected acrylic acid to at least one step selected from the group consisting of the crystallization step, the distillation step, and the absorption step.
That is, we have found that purifying the crude acrylic acid by use of the crystallization step can prevent the acrylic acid from further being dimerized, oligomerized, and polymerized, and can efficiently concentrate the dimer of acrylic acid, and subjecting the obtained concentrated dimer to the dimer decomposition step can easily collect the acrylic acid.
That is, the present invention relates to a method of producing acrylic acid, comprising the steps of oxidation, absorption, distillation purification, crystallization, and dimer decomposition, or the invention relates to a method for producing acrylic acid, comprising the steps of
a) catalytic vapor phase oxidizing propylene and/or acrolein with a molecular oxygen containing gas;
b) introducing the obtained gaseous mixture to an acrylic acid absorption column, and contacting the mixture with a solvent to absorb the mixture as an acrylic acid solution;
c) subjecting the acrylic acid solution to distillation step to separate a crude acrylic acid substantially not containing solvent from the solution as a column bottom stream and/or a column side stream;
d) feeding the crude acrylic acid to a crystallization step to melt crystallize the acrylic acid and separate the acrylic acid into a purified acrylic acid and a residual mother liquid; and
e) feeding the residual mother liquid to an acrylic acid dimer decomposition step to obtain a distillate, and feeding the distillate to at least one step selected from the group consisting of the crystallization step, the distillation step, and the absorption step.
The method of producing acrylic acid according to the present invention makes it possible to obtain the highly purified acrylic acid in a high yield.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.